In connection with CDMA communication systems and Wideband CDMA (WCDMA) communication systems, Heterogeneous Networks (HetNets) have recently been discussed and developed intensively. Heterogeneous networks concerns effects associated with networks where different kinds of cells are mixed. A problem is then that these cells may have different radio properties in terms of e.g. radio sensitivity, frequency band, coverage, output power, capacity or acceptable load level. This can be an effect of the use of different RBS sizes (macro, micro, pico, femto), different revision (different receiver technology, SW quality), different vendors and of the purpose of a specific deployment.
One of the most important factor in HetNets is that of air interface load management, i.e. the issues associated with the scheduling of radio resources in different cells and the interaction between cells in terms of inter-cell interference. The aspects that are of interest for the present disclosure are the algorithmic architectures associated with such air-interface load management in the UpLink (UL) of e.g. the WCDMA system. The reason for this renewed interest includes the need to optimize performance in HetNets and the fact that the concept of load changes with the introduction of advanced receivers.
To exemplify these problems, consider FIG. 1. That figure shows a Het Net 1 comprising a number of high power cells 2, also denoted as macro cells. A low power cell 4 with limited coverage is intended to serve a hotspot 3. In order to get a sufficient coverage of the hotspot 3 an interference suppressing receiver may be used. The problem is now that the low power cell is located in the interior of and at the boundary of a specific macro cell. Further, surrounding macro cells interfere with the low power cell rendering a high level of neighbor cell interference in the low power cell, that despite the advanced receiver reduces the coverage to levels indicated by reference numeral 5 that do not allow a major coverage of the hotspot 3. As a result, users of the hotspot 3 are connected to the surrounding macro cells 2, thereby further increasing the neighbor cell interference experienced by the low power cell 4.
From this discussion it should be clear that it would be advantageous if the Radio Network Controller (RNC) or the surrounding Radio Base Stations (RBSs) could be informed of the interference situation and take action, using e.g. admission control in the RNC or new functionality in the surrounding RBSs to reduce neighbor cell interference and to provide a better management of the hot spot traffic—in terms of air interface load. This requires RBS means to estimate the neighbor cell interference. This requires careful consideration of the algorithmic architectures involved.
Load estimation and load prediction as concepts are well know in prior art. Some of the applied methods are summarized in Appendix A.
Neighbor cell interference estimation as such based on uplink power measurements is known in prior art and it is e.g. described in [2]. The technology is based on the assumption that the powers of all radio links are measured in the uplink receiver. This is not the case today. The power measurement is associated with difficulties since the transmission of the WCDMA uplink is not orthogonal, a fact that causes errors when the powers are estimated and since the individual code powers are often small, making signal to noise ratios low as well. This fact contributes to the inaccuracy of said power estimates.
The major problem associated with the solution of [2], [4] is however that the sum of neighbor cell interference and thermal noise power needs to be estimated by means of high order Kalman filtering. This step has a very high computational complexity. Although [5] provides techniques that reduce this computational complexity it is still high when the number of users increase. The subsequent steps of [2] and [4] estimate the thermal noise power floor according to the prior art algorithms described above, followed by a subtraction to arrive at the neighbor cell interference.
The reference [3] recognizes a problem associated with power scaling of the output of the Kalman filter of the Rise over Thermal (RoT) estimator, i.e. the first block 301 of FIG. 2. Essentially, the problem is due to the fact that the Kalman filter is designed at a specific operating point in the linear power domain. Now, with recent traffic increases, the power operating point will vary significantly. With such significantly varying power operating point, the linear power domain of the Kalman filter makes the calculations non-optimized. Furthermore, these problems become even more severe when in-band interference external to the WCDMA system is entered into the equation. This is because such interference affects the UL power level significantly.
A problem is thus that inter-cell interference power estimation algorithms do not include functionality to handle noise floor and interference power variations anticipated in tomorrow's HetNets in an accurate manner.